Artificial islands, causeways, breakwaters and other civil engineering structures are constructed using well-known underwater fill techniques. Typically, particulate fill material such as sand or sand-silt is excavated at an underwater site termed the "borrow pit" and then dumped at an underwater construction site at which the particular structure is desired. Usually, very large quantities of fill material must be excavated and dumped at the underwater construction site in order to accumulate an adequate base to support whatever structures may be desired above the surface of the water. For example, if it is desired to build a causeway, then sufficient underwater fill must be dumped at the construction site to form an above-water surface upon which a road or railbed of desired width may be constructed. Similarly, if it is desired to build an artificial island, then sufficient fill must be deposited at the underwater construction site to yield an island having an above-water surface area adequate to support whatever buildings or equipment may be required to satisfy the contemplated end use of the island.
Masses of particulate fill dumped at an underwater site tend to form underwater fill piles shaped like truncated cones. It has been found that the side slopes of such piles typically form an angle of about three to five degrees with respect to the horizontal. It is the relatively shallow side slope inclination of the underwater fill pile which necessitates dumping massive quantities of fill material at the underwater construction site in order to accumulate an underwater base which will yield the desired above-water surface area.
Suppose, for example, that it is desired to construct an artificial island in fifteen meters of water such that the island surface is circular, one-hundred meters in diameter and projects two meters above the surface of the water. It can easily be shown that about 1,300,000 cubic meters of fill material would be required to construct the island if its sides slope at about five degrees with respect to the horizontal. Approximately three to four months would be required to construct the island using conventional dredging techniques.
Clearly, the time required to construct an island or other underwater structure could be reduced if the amount of fill material required could be reduced. Reduced construction time is of particular interest with respect to Arctic construction projects since climatic conditions in the Arctic permit construction operations to be carried out for at most three months of the year. The cost of constructing an artificial island (or other underwater fill) could also clearly be reduced by reducing the amount of fill material consumed. Concomitant environmental advantages may also be obtained by reducing the amount of fill material consumed, since the size of the borrow pit could be minimized, along with the surface area of the underwater bed upon which fill material must be deposited to yield the desired structure.
Obviously, the amount of material required to construct an underwater fill may be reduced so as to yield the aforementioned advantages by steepening the angle of inclination of the pile side slopes with respect to the horizontal. For example, if the artificial island discussed in the above example could be constructed with side slopes angled at fifteen degrees with respect to the horizontal, then only about 370,000 cubic meters of fill material would be required. Stated another way, less than one-third as much fill material, requiring only about one-third as much time to excavate and deposit, would be required to construct the island with fifteen degree side slopes as opposed to five degree side slopes. If the island was to be constructed in thirtyfive meters of water (as opposed to fifteen meters of water), then an island with fifteen degree side slopes would consume less than twenty percent as much fill material and construction time as an island having five degree side slopes. Indeed, the amount of fill and time required to construct an island having fifteen degree side slopes in thirty-five meters of water is roughly the same as would be required to construct a conventional island (having five degree side slopes) in only fifteen meters of water.
The difficulty, of course lies in overcoming the natural tendency of particulate fill material to accumulate in piles having three to five degree side slopes when deposited underwater during conventional construction operations. The state of the art is such that it is currently considered impractical to use underwater fill techniques to construct artificial islands in Arctic water deeper than about fifteen meters, since the shallow (i.e. about three to five degree) angle of inclination of the fill pile side slopes necessitates excavation and dumping of more material than can be handled during the short Arctic construction season. Where large working platforms are required in deeper Arctic water (i.e. up to about thirty-five meters), current practice is to utilize a costly structure (about $150,000,000) resting on a sand filled berm about ten meters high formed with conventional underwater fill techniques.
The present invention provides a method and apparatus for constructing an underwater fill which overcomes the foregoing disadvantages, facilitating construction of underwater fills having side slopes inclined at angles significantly greater than five degrees with respect to the horizontal, thereby greatly reducing the amount of fill material and time required.